1. Field of the Invention
The present invention relates to a suspension control system, especially to a suspension control system which alters a suspension characteristic suitably for conditions of the road on which the vehicle is running.
2. Description of the Related Art
A suspension control system has been proposed which includes a vehicle height sensor for detecting the changes in the distance between a vehicle axle and a vehicle body and which damps the vibration exerted on the vehicle body, in the case of rough road running, by altering the suspension characteristic as well as vehicle height in order to get improve ride comfort, control, and stability. An example of the above invention is Published Unexamined Japanese Patent Application No. 57-172808 entitled "Vehicle Height Adjusting Apparatus". It prevents bottoming of the vehicle due to its bounding by raising the vehicle height higher than a predetermined value, when a smooth road running changes into a rough road one.
However, the suspension control system according to the prior art possesses the following problems.
A problem is the slowness in determining a continuous rough road, since it has been executed by counting the frequency at which an actual vehicle height is different from an average vehicle height by a predetermined value or more within a predetermined time interval. Inconvenience is thus quite probable that the vehicle has already moved out of the rough road onto a smooth road, and the reliability of detecting a rough road is lowered.
Another problem is, relating to the above-mentioned slow determination of the rough road, the inability to quickly damp cyclical vibrations such as pitching or bouncing which follows the rough road running by altering the suspension characteristic. Therefore, the suspension control system according to the prior art, allowing the vibration to continue and the vehicle position to change, has the disadvantages such as reduced control and stability as well as discomfort for the passengers.
A further problem is the reduced durability and reliability due to an unnecessarily frequent suspension characteristic alteration. Once the suspension characteristic is altered, the vibrating condition of the vehicle changes even if the vehicle runs on the road of the same conditions as it has. However, the condition for determining the continuous rough road is not corrected in response thereto, thus the hunting of the control for altering the suspension characteristic is caused.
Further more, the suspension control system according to the above-mentioned prior art determines the rough road running when a vehicle height change value gets greater than a reference value more often than a predetermined frequency, thus causing the following problems.
A problem is the reduced durability and reliability of a vehicle height adjustment means. A vehicle height, for example, is altered frequently even in response to a sporadic bump or dip on the road surface, unless the condition for raising the vehicle height is set adequately.
Another problem is the reduced control and stability, with the vehicle body rolling, during high speed. This is because the center of balance of the vehicle gets higher since the vehicle height is raised frequently.
A further problem is the difficulty both in detecting correctly the condition of rough road running and in adjusting the vehicle height, when the condition for those operation is the same as that for altering the suspension characteristic.